Internally threaded coil form for securing threaded core therein



April 14, 1964 3,129,397

INTERNALLY THREADED COIL FORM FOR SECURING 4 w 24 i fi B V g w w 1 M i y a i w 3 T s i i w 2; 4 1 mg United States Patent Ollice 3,129,397 Patented Apr. 14, 1964 3,129,397 INTERNALLY THREADED COIL FORM FOR SECURING THREADED CORE THEREIN George A. Scherry, Berkeley, IlL, assignor, by mesne assignments, to Grayhill Moidtronics, Inc., Clarendon Hills, Ill., a corporation of Illinois Filed Oct. 27, 1960, Ser. No. 65,527 6 Claims. (Cl. 336-136) This invention relates generally to electrical coil devices, and more particularly to such a device having a miniature coil form adapted to receive a core member.

It is common practice in the manufacture of inductance coils to provide a supporting member or form on whichthe inductance coil is wound. This supporting member may have an internally threaded center hole adapted to receive a cylindrical, externally threaded core member. Such cores are made of powdered iron or other material so as to vary the inductance of the surrounding coil winding. A threaded engagement between the coil form and the core is preferred as a convenient way of holding the core in place and also to obtain axial movement of the core for fine adjustment of the inductance of the surrounding coil. Both qualities are enhanced by increasing the fineness of the thread gauge.

However, this arrangement presents serious problems in the manufacture of miniature coil assemblies. The known materials preferred for cores because of their electrical characteristics are often hard and brittle. Consequently, they are difiicult to machine and the fine gauge threads are easily damaged.

It has been proposed to provide a longitudinal slot or groove in the core and to insert therein a soft, flexible or readily deformable material, either in lieu of threading the core or to compensate for a loose engagement of the core threads with the internal coil form threads. This solution is costly since it requires extra machining of the core, and, in addition, assembly and afiixing of the flexible material on the core. Should the flexible material not be glued or otherwise attached to the core, it is subject to the further problems of working loose and becoming lost when the threaded members are separated. Miniaturization increases the severity of these problems, since, as the overall size of the coil assembly is reduced, it becomes more diflicult and expensive to perform machining operations on threaded cores and to construct insulated coil forms and terminations for the coil windings.

Accordingly, it is an object of the present invention to provide an improved coil assembly in which a core is held in secure engagement with a coil form Without the necessity of modifying the core.

Another object is to provide a coil form which is simple in construction and which receives and securely engages an adjustable core without danger of damage thereto.

A further object of the invention is to provide a simple coil form which can be molded of an insulating material.

A feature of the invention is the provision of a miniature coil assembly in which the coil supporting member carries an internal thread adapted to receive a threaded core for axial adjustment therein, and an insert member is provided in an axially offset longitudinal recess in the supporting member to resiliently engage the core.

Another feature of the invention is the provision of a miniature molded insulating coil form with flange portions at its ends for confining the coil windings and having an internally threaded center hole adapted to receive a threaded core member. The flange portions are each provided with axially offset recesses which open into the center hole, and cylindrical inserts which may be made of nylon are press fit therein so that a portion thereof engages the threads of the core member to hold the core securely in the coil form, without danger of either the core member or the inserts working loose.

In the accompanying drawings:

FIG. 1 is a greatly enlarged sectional side view of a coil assembly in accordance with the invention taken on line 1-1 of FIG. 2;

FIG. 2 is a greatly enlarged end view of the coil assembly; and

FIG. 3 is an exploded perspective view illustrating the relationship of component parts of the coil assembly.

Briefly, the invention provides an improved coil assembly including a molded insulating hollow coil form of the type having a longitudinal center hole adapted to threadably receive a cylindrical core member. One or more coils may be wound on the coil form. The coil form is provided with a recess extending axially inward from an end face of the coil form. The recess is offset from the axis of the center hole by a predetermined distance so that the peripheral surfaces of the recess and center hole intersect to form an opening therebetween. An insert, made of resilient material such as nylon, is shaped so that it is received in the recess with a press fit. A portion of the insert protrudes through the opening and into the center hole to resiliently engage the threads of the core member, thereby holding the same in fixed position. The axial arrangement permits maximum engagement between the insert and the threaded core to require a minimum of space. The resulting coil form can be simply and economically molded with terminal and/ or mounting flanges thereon.

The coil assembly shown in FIG. 1 is greatly enlarged to clearly illustrate the invention. The overall length of a representative coil form is only 1%; inches or roughly the size of an ordinary paper clip. The coil form includes an elongated tubular coil receiving portion 11 which serves as the form on which one or more electrical coils 20 may be wound. It has generally square flange portions 12 and 13 at either end which may confine the ends of the coils. Flange portions 12 and 13 also serve as mounting footings for the coil form and have a plurality of conducting terminals 14 imbedded therein for connection to the coils. The miniature coil form may be molded from thermo-setting plastic selected from any of a variety of the basic resins and fillers such as phenolic, urea, melamine and others.

The coil form has a hollow interior forming a longitudinal center hole 16 which may be threaded all the way through to provide an internal screw thread 17. A cylindrical core member 18, which may have a square shaped center hole 19, is provided with an external thread of the proper size and gauge to be threadably received in the center hole. The coil form may be used for coils of various types which may include a single winding or a plurality of windings, and the windings may be positioned side by side along the form, or as layers one over the other. In the coil assembly illustrated, two coils 20 are provided on the form which are substantially the same length along the form. The core 18 is slightly less than half the length of the coil form, and two of such cores may be inserted in the coil form to control the inductance of the two coils wound on the form. The cores may be composed of highly permeable material, such as powdered iron, or of conducting material, to provide an ad justable flux path for the inductance coil.

The coil form may include flange portions 12 and 13 at either end thereof as shown, or a single flange on one end. The flange portions are provided with recesses 21 and 22 respectively, which may be cylindrically shaped (FIG. 2) and which extend for a given distance longitudi nally into the flange portions and parallel with the axis of center hole 16. The axes of recesses 21 and 22 are offset-from the axis of center hole 16 by a predetermined distance (FIG. 3) so that the peripheral surface of each recess intersects the peripheral surface of hole 16 to provide an opening therebetween.

Shaped inserts made of resilient materiaL'such as cylindrical nylon tubes 23 and 24, are received respectively in each recess 21 and 22 with a press fit. For this purpose the diameters of inserts 23 and 24 are made slightly oversize with respect to the diameters of the recesses. Thus a portion of the peripheral surface of each insert projects into center hole 16 by a predetermined amount depending on the aforementioned axial offset of the recesses (FIG. 2).

In assembling a coil form embodying the invention, the inserts 23 and 24 are first fully pressed into their respective recesses 21 and 22 so as to be slightly recessed from the end faces of flange portions 12 and 13. The interference fit of the inserts insures that they will remain in place after having been once inserted. Then magnetic core 18 is screwed into the threaded center hole 16 by means of a wrench adapted to fit into the square shaped center hole 19 thereof. The core is easily started on threads 17 since the inserts are recessed from the end faces of the flanges so as not to interfere with the first turn or two of the internal threads. As the core 18 is further screwed into the center hole the external threads of the core engage the interfering outer surface of insert 23 in a resilient contact which insures a tight engagement between the core and the coil form.

When the inserts are made of nylon, or a similar material, and the core made of powdered iron, the harder core material will tend to cut grooves in the interfering portion of the insert surface corresponding to internal threads 17. However, the resilient property of nylon will prevent any damage to the external threads of core 18. The interference fit provided by the insert also permits larger tolerances to be used between the pitch diameters of the external threads of the core and the internal threads 17 without danger of the core working loose from the coil form. The axial engagement of the insert with the core provides a substantially constant force which resists movement of the core, but which can be overcome to provide the required smooth and fine adjustment of the core position.

In one embodiment constructed in accordance with the above description, the following materials and dimensions were found to successfully fulfill the stated objects of the invention, but it is not intended to limit the invention thereto:

Coil form material Diallyl phthalate. Insert material Nylon. Core material Ferrite. Core thread form 8-32 (Nat. Std.). Pitch diameter of core thread .1424 inch. Diameter of recess .094 inch. Distance between axes of recess and center hole .119 inch.

coil form and the coil or coils thereon.

I claim:

1. A coil form assembly for supporting an inductance coil comprising an elongated member having an internally threaded longitudinal center hole adapted to receive an externally threaded core for axial adjustment therein, said elongated member having a coil receiving portion and a radially extending flange portion at one end thereof, said flange portion having an elongated recess therein, said recess extending from the outer end of said member substantially parallel to said center hole and axially offset therefrom, with the peripheral surface of said recess intersecting the peripheral surfaces of said center hole to form an opening therebetween, and a resilient insert positioned in said recess and having a press fit therein with a portion of said insert projecting into said center hole for resilient interference engagement with the external threads of the core to hold the core and permit fine adjustment thereof.

2. A coil form assembly as defined in claim 1 wherein a said recess in said flange portion is cylindrical, and said insert is of tubular configuration.

3. A coil form assembly for supporting an inductance coil including in combination, an elongated member having an internally threaded longitudinal center hole, said elongated member having a coil receiving portion and an integral radially extending flange portion at one end thereof, said flange portion having a longitudinal recess in the end face thereof, said recess being parallel to and axially offset from said center hole by a predetermined distance with the peripheral surface thereof intersecting the peripheral surface of said center hole to form an opening therebetween, a tubular nylon insert in said recess and having a press fit therein and being inset from the face of said flange portion, and an externally threaded magnetic core threadably received in said center hole for axial adjustment therein, with a portion of said insert projecting into said center hole to resiliently engage the external threads of said magnetic core to securely hold the same in position.

4. A coil form assembly as defined in claim 3 wherein said magnetic core is formed of brittle magnetic material and has threads with rounded apices.

5. A miniature molded plastic coil form assembly for supporting an inductance coil including in combination, an elongated tubular member having an internally threaded longitudinal center hole for receiving a threaded magnetic core therein, said tubular member having a coil receiving portion and first and second radially extending integral flange portions at the ends thereof for confining the inductance coil, said flange portions each having a longitudinal cylindrical recess in the end faces thereof, said recesses being parallel to and axially olfset from said center hole by a perdetedmind distance with the peripheral surface thereof intersecting the peripheral surface of said center hole to form an opening therebetween, and first and second tubular nylon inserts received respectively with a press fit in said recesses of said first and second flange portions and being inset from the end faces thereof, with a portion of said inserts projecting into said center hole to resiliently engage the external threads of the magnetic core to securely hold the same in place in the presence of vibration and permitting smooth fine adjustment of the position of such core.

6. A molded plastic coil form assembly for supporting an inductance coil including in combination, an elongated tubular member having an internally threaded longitudinal center hole for receiving threaded magnetic cores therein, said tubular member having a coil receiving portion and first and second radially extending integral flange portions at the ends thereof for confining the inductance coil, said flange portions each having a longitudinal cylindrical opening in the end faces thereof, said openings being parallel to and axially offset from said center hole by a predetermined distance with the peripheral surface thereof intersecting the peripheral surface of said center hole to form an opening therebetween, and first and second tubular nylon inserts received respectively in said openings of said first and second flange portions and being inset from the end faces thereof, with a portion of said inserts projecting into said center hole to resiliently engage the external threads of the magnetic cores to securely hold the same 5 6 in place in the presence of vibration and permitting smooth FOREIGN PATENTS fine adjustment of the position of such cores. 972,647 France Dem 27, 1948 1,054,176 Germany Apr. 2, 1959 References Cited in the file of this patent UNITED STATES PATENTS 5 2,581,165 Bonanno Jan. 1, 1952 

1. A COIL FORM ASSEMBLY FOR SUPPORTING AN INDUCTANCE COIL COMPRISING AN ELONGATED MEMBER HAVING AN INTERNALLY THREADED LONGITUDINAL CENTER HOLE ADAPTED TO RECEIVE AN EXTERNALLY THREADED CORE FOR AXIAL ADJUSTMENT THEREIN, SAID ELONGATED MEMBER HAVING A COIL RECEIVING PORTION AND A RADIALLY EXTENDING FLANGE PORTION AT ONE END THEREOF, SAID FLANGE PORTION HAVING AN ELONGATED RECESS THEREIN, SAID RECESS EXTENDING FROM THE OUTER END OF SAID MEMBER SUBSTANTIALLY PARALLEL TO SAID CENTER HOLE AND AXIALLY OFFSET THEREFROM, WITH THE PERIPHERAL SURFACE OF SAID RECESS INTERSECTING THE PERIPHERAL SURFACES OF SAID CENTER HOLE TO FORM AN OPENING THEREBETWEEN, AND A RESILIENT INSERT POSITIONED IN SAID RECESS AND HAVING A PRESS FIT THEREIN WITH A PORTION OF SAID INSERT PROJECTING INTO SAID CENTER HOLE FOR RESILIENT INTERFERENCE ENGAGEMENT WITH THE EXTERNAL THREADS OF THE CORE TO HOLD THE CORE AND PERMIT FINE ADJUSTMENT THEREOF. 